Synthetic Aperture Radar (SAR) & NASA-ISRO SAR (NISAR) Mission
What is SAR?
SAR uses microwave pulses (instead of visible light) to create high-resolution images, even in darkness or cloudy weather.
A small moving antenna (on a satellite/aircraft) records echoes from the ground, and advanced processing combines them to simulate a large virtual antenna, improving resolution.
Advantages: Works 24/7, penetrates clouds/smoke, detects changes in soil, water, ice, and man-made structures.
NASA-ISRO SAR (NISAR) Mission
On June 12, 2025, the NISAR satellite arrived at ISRO’s Sriharikota spaceport for launch.
Once operational, NISAR will:
Scan nearly all of Earth’s land and ice surfaces twice every 12 days.
Provide unprecedented environmental data (e.g., deforestation, glaciers, disasters).
SAR Antenna Mechanism :
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Antenna Role:
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Central to SAR; it receives the microwave echoes bounced back from the surface.
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Resolution Dependence:
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Normally, longer antennas provide better image resolution.
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Practical Challenge:
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Constructing and operating large physical antennas in space is difficult.
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SAR Solution:
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Uses a smaller antenna mounted on a moving platform (e.g., satellite or aircraft).
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Synthetic Aperture Concept:
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As the platform moves, echoes are captured from different positions.
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Each echo carries slightly different spatial information.
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Signal Processing Technique:
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Collected echoes are combined using precise timing and phase data.
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This mimics a much larger antenna — sometimes hundreds of metres long — synthetically.
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Outcome:
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Achieves high-resolution imaging with a physically compact antenna setup.
Advantages of Synthetic Aperture Radar (SAR)
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All-Weather Capability:
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Microwaves can penetrate clouds, smoke, and light rain.
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Enables continuous data collection day and night, in all weather conditions.
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Wide Area Coverage:
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When placed on orbiting satellites, SAR can scan hundreds of kilometres of land in a single pass.
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Material Differentiation:
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Different surfaces (soil, water, vegetation, metal) reflect microwaves differently.
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Helps in distinguishing between various natural and man-made features.
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Change Detection:
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Can detect subtle surface changes (e.g., soil moisture, deforestation, flooding) that optical sensors may miss.
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High Temporal Resolution:
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Frequent revisits enable monitoring of dynamic changes like glacier movement, crop growth, or disaster impact.
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